The invention relates to a method for functionally checking a vacuum switch of a traction converter.
A traction converter of said type is illustrated in more detail in FIG. 1. Said traction converter has a four-quadrant chopper 2 on the AC supply side and a self-commutated converter 4, which is also referred to as a pulse-controlled converter, on the load side which are electrically connected in parallel on the DC voltage side by means of an intermediate circuit capacitor CZK. A three-phase AC motor 6 is connected at the outputs of the pulse-controlled converter 4. On the AC voltage side the four-quadrant chopper 2 can be connected in an electrically conductive manner to a contact wire 14 by means of a secondary winding 8 of a traction transformer 10 and a vacuum switch 12. One terminal of the secondary winding 8 of the traction transformer 10 in each case is connected in an electrically conductive manner to an AC voltage terminal 16 or 18 of the four-quadrant chopper 2. A single-phase AC supply voltage uN is converted by means of said four-quadrant chopper 2 into a predetermined intermediate circuit DC voltage UZK, from which a voltage system adapted to the needs of the three-phase AC motor 6, for example a three-phase voltage system of variable amplitude and frequency, is generated.
In said known embodiment variant of the traction converter, deactivatable power semiconductor switches, in particular Insulated Gate Bipolar Transistors (IGBTs), are used in each case as converter valves T1 to T6 of the self-commutated converter 4 and as converters T7 to T10 of the four-quadrant chopper 2. A power diode D1 to D6 or D7 to D10 is electrically connected in parallel with each deactivatable power semiconductor switch T1 to T6 or T7 to T10, respectively.
The power flow direction relevant to the converter configuration is from the feeding network by way of the contact wire 14 and the traction transformer 10, the four-quadrant chopper 2 and the self-commutated converter 4, to the three-phase AC motor 6. A feedback control device 20, to which the single-phase AC supply voltage uN is fed, is provided for controlling the converter valves T7 to T10 of the four-quadrant chopper 2. A microcontroller accommodated in the open- and closed-loop control device of the traction converter can be provided for example as the feedback control device 20.
A traction converter as illustrated in FIG. 1 is known from the publication titled “Comparison of Multi-System Traction Converters for High-Power Locomotives” PESC Conference 2004 from Jun. 21 to Jun. 23, 2004 in Aachen, in particular from FIG. 10 of said publication. If importance is attached to a low harmonic factor of the AC line current on the AC supply side, two four-quadrant choppers are provided on the AC supply side, each of which is connected in parallel on the AC supply side via a transformer secondary winding and operates on a common DC voltage intermediate circuit. Said two four-quadrant choppers are controlled by means of phase-shifted clocking of the two four-quadrant choppers. What is achieved thereby is that the harmonic components of the AC supply currents on the secondary side are electrically shifted relative to one another through 180° on the secondary side and consequently largely compensate one another on the primary side of the traction transformer through summation. A traction converter of said type is also known from the above-cited publication, in particular from FIG. 8 or FIG. 12 of said publication.
The vacuum switch 12 is a power switch 12 that is dimensioned for high currents. Said switch is able not only to switch on operating currents, but also to hold and switch off high overload currents and short-circuit currents for a predefined time in the event of faults. The contacts of the vacuum switch are contained in a vacuum in order to avoid arcing. Said vacuum switches are mainly used only in medium voltage installations having voltages up to approx. 40 kV and are suitable for very great switching frequencies while being virtually maintenance-free. Said vacuum switch can lose its functional capability as a result of spontaneous or gradual loss of the vacuum. In the event of a fault the entire traction converter can be affected as a result of a vacuum switch becoming inoperable in such a way.
It is well-known to use a test device in order to enable the functional integrity of a vacuum switch of said type to be checked. For this purpose the vacuum switch of the traction converter must be separated from the high-voltage equipment and the test device connected. Known test devices operate with a test voltage of several kilovolts and must be equipped with a high-voltage transformer and correspondingly dimensioned insulation. It is also necessary to observe extensive precautionary measures during the testing itself. The operating principle of the known test instrument provides that the vacuum chamber is tested with a test voltage lying between the breakdown voltage at full vacuum and the breakdown voltage of air at atmospheric pressure. Said test voltage is applied to the open switching contacts and the internal breakdown voltage is determined. In this way it is established whether the tested vacuum chamber still possesses a sufficient vacuum.
This testing of the vacuum switch represents investment of resources and downtimes of the traction drive. If the testing is not carried out, failure of the vacuum switch can result in high consequential damage. Since a test device has to be used for functionally checking the vacuum switch, said functional check can only be carried out in a railroad or streetcar depot.
The object underlying the invention is thus to disclose a method for functionally checking the vacuum switch of a traction converter, as a result of which the use of a test device can be dispensed with.